Effects of Thermal Annealing for Restoration of UV Irradiation Damage during Plasma Etching Processes

During plasma etching processes, UV and vacuum ultraviolet (VUV) photon irradiation generates defects and causes an increase in interface state density between SiO₂ and Si. To understand the effects of pulse-time-modulated plasma on reducing damage, defects in a SiO₂ film (E' center) after etching processes were measured using the electron spin resonance (ESR) method. We found that the density of the E' center was markedly reduced using C₄F₈/O₂ pulse-time-modulated (TM) plasma etching, as compared with continuous wave (CW) plasma etching. This corresponds to the changes in interface state density using the charge pumping method. Conversely, in both cases, these E' centers were almost eliminated by H₂/N₂ annealing at 400 °C. Interface state density, however, remained constant even by annealing in both cases, whereas, interface state density when using TM plasma was much smaller than that when using CW plasma. This result suggests that H₂/N₂ annealing is not sufficient to restore the interface state density between SiO₂ and Si. To reduce the increase in interface state density, UV irradiation damage must first be suppressed during plasma etching. Consequently, a combination of TM plasma etching and thermal annealing is very effective for reducing UV irradiation damage to achieve high reliability in metal–insulator–silicon (MIS) devices.